Technique for the growth of single crystal lead molybdate

ABSTRACT

Large single crystals of lead molybdate of high optical quality are grown by pulling from a melt under conditions of low thermal gradient in a direction approximately 30* off the c-axis. Conventional growth technique on either the a- or c-axis have not proven to yield satisfactory crystals from the standpoint of magnitude or yield.

United States Patent Bonner 51 Apr. 4, 1972 [54] TECHNIQUE FOR THE GROWTH OF SINGLE CRYSTAL LEAD MOLYBDATE [72] inventor: William Adam Bonner, Scotch Plains, NJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

[22] Filed: Aug. 3, 1970 21 Appl. No.: 60,332

[52] U.S. Cl. ..23/l5 W, 23/51, 23/301 SP [51] Int. Cl. ..C22b 59/00, B0 lj 17/18 [58] Field of Search ..23/301SP,15 W, 51 R [56] References Cited UNITED STATES PATENTS Kiffer et al ..23/301 3,328,311 6/1967 Borchardt ..23/51 3,346,344 10/ 1967 Levinstein et al..

3 ,446,603 5/1969 Loiacono et a1. ..23/ 301 OTHER PUBLICATIONS Zobnina, et a1., lzv. Akad. Nauk. SSSR, Neorg. Mater. 2( 12), 2199- 2203 (1966).

Primary Examiner-Norman Yudkoff Assistant ExaminerR. T. Foster Attorney-R. J. Guenther and Edwin B. Cave [5 7] ABSTRACT 5 Claims, 1 Drawing Figure TECHNIQUE FOR THE GROWTH OF SINGLE CRYSTAL LEAD MOLYBDATE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a technique for the growth of single crystal lead molybdate. More particularly, the present invention relates to a technique for the growth of single crystal lead molybdate of high optical quality by pulling from a melt under conditions of low thermal gradient.

2. Description of the Prior Art In recent years, the range of application of acousto-optic devices has been steadily expanding (see Volume 54, Proceedings of the IEEE page 1392, Oct. 1966). These devices are, in essence, elastic wave-induced three dimensional diffraction gratings producing an angular diffraction of a portion of an incoming electromagnetic wave. The angle of diffraction and the portion diffracted generally increase with the frequency and amplitude, respectively, of the interacting elastic wave. This mechanism naturally suggests position sensitive devices, such as, beam deflectors used, for example, in

information retrieval systems. Other uses may take advantage of ancillary effects as, for example, the variation in amplitude of the through-transmitted or diffracted beams due to variation in some property of the elastic wave.

Recognizing the design advantages of acousto-optic devices for certain uses, there has been considerable experimentation using a large number of materials and relevant characteristics have been reported, see, for example, Journal of Applied Physics, Volume 38, page 5149, 1967. For many purposes, the most promising acousto-optic material reported in the noted paper was lithium niobate. Power levels and bandwidth obtained with this material make it clear that more efficient materials are required.

A recently developed material, alpha iodic acid, aHlO has a substantially higher acousto-optic figure of merit than any of the earlier reported materials. This material is water soluble and can be grown from aqueous solution with high optical quality. However, due to the solubility, special fabrication procedures and protection from environmental constituents are required.

More recently, a class of substantially water insoluble materials, including PbMo0,, TiRe0 and PbW0 has been found to manifest significantly higher acousto-optic figures of merit than earlier investigated water insoluble materials. While figures of merit are in some cases somewhat inferior to that of alpha iodic acid, these materials are of importance because of practical advantages, such as ease of fabrication, because there is no need to protect devices from atmospheric constituents. Within this class of compounds lead molybdate is perhaps the most valuable, not only because diffraction angle is independent of polarization, but also because it is easily grown, manifests a high acousto-optic figure of merit, low acoustic and optical loss below IOGI-Iz, and favorable mechanical impedance for acoustic matching.

Nonetheless, conventional growth techniques have not been entirely satisfactory for the growth of this material from the standpoint of crystal size and yield, so resulting in a continuing investigatory program calculated to overcome these limitations. It has been found that single crystal lead molybdate grown in the a-direction shatters easily, particularly during attempts to anneal, and is excessively strained as a result of an unequal radial coefficient of expansion in the plane normal to the grown direction. Crystals grown in the c-direction have typically been small in size and, as previously noted, limited from the standpoint of yield. Accordingly, workers in the art have long sought a solution to these difficulties.

SUMMARY OF THE INVENTION gradient in a direction approximately 30 off the c-axis. Large single crystal lead molybdate eminently suited for acoustooptic device applications has been grown in this manner.

BRIEF DESCRIPTION OF THE DRAWING The invention will be more readily understood by reference to the accompanying drawing wherein:

The FIGURE is a front elevational view partly in cross-section of a typical apparatus utilized in the practice of the present invention.

DETAILED DESCRIPTION With reference now more particularly to the FIGURE, there is shown an apparatus suitable for use in growth of lead molybdate in accordance with the present invention. Shown in the FIGURE is a double shell water cooled growth chamber 11 having disposed therein a fused silica liner 12 disposed upon pedestal 12A and fitted through the work coil 13 of an r-f generator 14, coil 13 being connected to generator 14 by means 15. Liner 12 contains a suitable powder 16, typically monoclinic zirconia powder, in which is embedded crucible susceptor 17 from which growth is to be effected. In this apparatus thermal gradients are minimized by reflection of radiation back to the melt by the conical top and uncontrolled fluctuations are avoided by the use of a sealed chamber. Cooling is effected by circulation of water between the shells of the apparatus, water being admitted thereto via inlet 18 and removed in an outlet 19. Seed holder or rod 21 connected to shaft 22 which is operated by motor 23 serves as the site of crystal growth upon a seed crystal 24. During the growth process rod 21 is elevated by mechanical means 25.

In the operation of the process, reagent grade lead oxide and molybdenum trioxide obtained from commercial sources are mixed in equimolar proportions in quantities sufficient to fill the crucible selected for growth purposes, crystal size being dependent upon the volume of the melt. The mechanically mixed components are next introduced into a suitable crucible susceptor, typically platinum, and with water flowing through the cooling channel of chamber 11, in an apparatus similar to that shown in the FIGURE. The r-f coil 13 is activated by means of r-f generator 14, thereby resulting in the formation of a melt at a temperature of the order of 1 C. Then, a lead molybdate seed cut 30 off the c -axis is affixed to one end of rod 21 and with chamber 11 sealed is slowly lowered to the melt surface and permitted thereafter to melt back sufficiently to clean the surface. Following, with rod 21 rotating at a speed within the range of 20 to 40 revolutions per minute, the seed crystal is lifted from the melt at a rate ranging from one-fourth to one-half inch per hour.

Optimum growth conditions have been found to exist in air under normal atmospheric pressure using a rotation rate of approximately 40 rpm and a pull rate of one-fourth inch per hour when the growth direction is as indicated.

Next, the grown crystal is subjected to an anneal at a temperature within the range of 750 to 1050 C. for a time period of at least 1 hour. The lower temperature limit is dictated by considerations relating to the relief of strain, whereas the upper temperature limit is dictated by the melting point of lead molybdate. The 1 hour minimum annealing time is that period required to bring the lead molybdate to equilibrium. Finally, the annealed crystal is cooled at a rate within the range of 5 to 10 C. per hour. At cooling rates greater than the noted maximum strain is reintroduced into the grown crystal, the shorter time period being dictated by practical considerations.

Exemplary embodiments of the present invention are set forth below. These examples and the foregoing description are intended to be illustrative in nature only and are not to be construed as restrictive.

EXAMPLE I 111.6 grams of lead oxide and 71.9 grams of molybdenum trioxide were mechanically mixed and introduced into a platinum crucible in an apparatus similar to the shown in the FlGURE. Then, the r-f coil was energized by the r-f generator and melting of the mixture effected at approximately 1 100 C. Then, with water flowing through the cooling channel of the growth chamber, a seed crystal comprising lead molybdate cut 30 off the c-axis was afiixed to the seed holder of the apparatus, lowered into the melt and permitted to melt back. Following, the seed crystal was rotated at 40 r.p.m. and pulled from the melt at a rate of approximately one-fourth inch per hour. Growth was continued at a temperature sufficient to maintain constant diameter for a time period sufficient to consume approximately two-thirds of the melt. After growth, the crystal was annealed at 800 C. for 2 hours and cooled at 5 C. per hour to room temperature, so resulting in the formation of a crystal of high optical quality 1 inch in diameter by 3 inches in length and weighing 200 grams, The criteria used for evaluating the optical quality was the change in spot diameter of a 6328 A helium-neon laser source. No unreasonable change was observed for a one centimeter path length in the direction of the c -axis.

EXAMPLE II The procedure of Example I was repeated with the exception that the platinum crucible was 2 inches in diameter and 2 inches high and the melt was comprised of 334.8 grams of lead oxide and 215.8.grams of molybdenum trioxide. The resultant lead molybdate crystal was 1% inches in diameter and 5 inches in length and of high optical quality.

I claim:

1. Technique for the growth of single crystal lead molybdate comprising the steps of (a) mixing equimolar proportions of PbO and M00: and introducing the resultant mixture to a suitable crucible, (b) melting said mixture, (c) introducing a lead molybdate seed crystal into the resultant melt, (d) pulling the seed crystal from the melt in a direction approximately 30 off the c-axis, and (e) subjecting the resultant pulled crystal to an anneal at a temperature within the range of 750 to 1050 C. for at least 1 hour.

2. Technique in accordance with claim 1 wherein' the seed crystal is rotated at a rate within the range of 20 to 40 revolutions per minute and lifted at a rate within the range of onefourth to one-half inch per hour during the pulling stage.

3. Technique in accordance with claim 1 wherein subsequent to annealing the pulled crystal is cooled at a rate within the range of 5 to 10 C. per hour.

4. Technique in accordance with claim 2 wherein the said crystal is rotated at 40 revolutions per minute and lifted at a rate of one-fourth inch per hour.

5. Technique in accordance with claim 4 wherein the crystal is annealed at 800 C. for 2 hours. 

1. Technique for the growth of single crystal lead molybdate comprising the steps of (a) mixing equimolar proportions of PbO and Mo03 and introducing the resultant mixture to a suitable crucible, (b) melting said mixture, (c) introducing a lead molybdate seed crystal into the resultant melt, (d) pulling the seed crystal from the melt in a direction approximately 30* off the c-axis, and (e) subjecting the resultant pulled crystal to an anneal at a temperature within the range of 750* to 1050* C. for at least 1 hour.
 2. Technique in accordance with claim 1 wherein the seed crystal is rotated at a rate within the range of 20 to 40 revolutions per minute and lifted at a rate within the range of one-fourth to one-half inch per hour during the pulling stage.
 3. Technique in accordance with claim 1 whErein subsequent to annealing the pulled crystal is cooled at a rate within the range of 5* to 10* C. per hour.
 4. Technique in accordance with claim 2 wherein the said crystal is rotated at 40 revolutions per minute and lifted at a rate of one-fourth inch per hour.
 5. Technique in accordance with claim 4 wherein the crystal is annealed at 800* C. for 2 hours. 